I. Field of the Invention
This invention relates to vessels used for containing and/or conveying molten metals and, especially, to such vessels having two or more refractory lining units that come into direct contact with each other and with the molten metals during use. More particularly, the invention addresses issues of molten metal leakage and thermal optimization in such vessels.
II. Background Art
A variety of vessels for containing and/or conveying molten metals are known. For example, molten metals such as molten aluminum, copper, steel, etc., are frequently conveyed through elongated troughs (sometimes called launders, runners, etc.) from one location to another, e.g. from a metal melting furnace to a casting mold or casting apparatus. In recent times, it has become usual to make such troughs out of modular trough sections that can be used alone or joined together to provide an integral trough of any desirable length. Each trough section usually includes a refractory liner that in use comes into contact with and conveys the molten metal from one end of the trough to the other. The liner may be surrounded by a heat insulating material, and the combined structure may be held within an external housing or shell made of metal or other rigid material. The ends of each trough section may be provided with an enlarged cross-plate or flange that provides structural support and facilitates the connection of one trough section to another (e.g. by bolting abutting flanges together).
It is also known to provide metal conveying troughs with heating means to maintain the temperature of molten metal as it is conveyed through the trough, and such heating means may be positioned within the housing close to an external surface of the refractory liner so that heat is transferred through the liner wall to the metal within. For example, U.S. Pat. No. 6,973,955 which issued on Dec. 13, 2005 to Tingey et al. discloses a trough section having an electrical heating element beneath the refractory liner held within an external metal housing. In this case, the refractory liner is made of a material of relatively high heat conductivity, e.g. silicon carbide or graphite. A disadvantage noted for this arrangement is that molten metal may leak from the liner (e.g. through cracks that may develop during use) and cause damage to the heating element. To protect against this, a metal intrusion barrier is provided between the bottom of the refractory liner and the heating element. The barrier may take the form of a screen or mesh made of a non-wettable (to molten metal) heat-resistant metal alloy, e.g. an alloy of Fe—Ni—Cr. While the molten metal intrusion barrier of the above patent can be effective, it is usually difficult to install in such a way that all of the molten metal resulting from a leak is prevented from contacting the heating element. Also, this solution to the problem of metal leakage tends to be expensive, particularly when exotic alloys are employed for the barrier.
The problem of molten metal leakage from the refractory liner is increased when the liner itself is made up of two or more liner units abutted together within a trough or trough section. The joint between the two liner units forms a weak spot where metal may penetrate the liner. The use of two or more such units is necessary in many cases because there is a practical limit to the lengths in which the refractory liner units can be made without increasing the risk of cracking or mechanical failure, but trough sections longer than this limit may be necessary to minimize the number of sections required for a complete trough run. When a trough section contains two or more refractory liner units joined end to end, the units are generally held together with compressive force (provided by the housing and end flanges) and the intervening joint is commonly sealed only with a compressible layer of refractory paper or refractory rope. Over time, such seals degrade and an amount of molten metal commonly leaks through the liner into the interior of the housing. If the trough section contains one or more heating elements or other devices, the molten metal will often find its way to such heating elements or devices and cause equipment damage and electrical shorts.
A further disadvantage of known equipment is that, when heated troughs or trough sections are utilized, a refractory lining of high heat conductivity is generally utilized to allow efficient heat transfer through the refractory material of the trough liner. However, this can have the disadvantage that heat is conducted along the refractory liner to the metal end flange, thereby creating a region of high heat loss from the liner and a hazardous region of high temperature on the exterior of the housing.
Accordingly, there is a need for improvement of trough sections of this general kind in order to address some or all of these problems and possibly additional issues.